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CHAPTER 1

INTRODUCTION AND LITERATURE REVIEW

1.1 Fundamentals of adhesives

Adhesives and sealants surround us in nature and in our daily lives. Substantial

businesses exist to develop, manufacture, and market these materials, and they are used

within virtually every business and industry. Applications abound from office to

automotive safety glass and from footwear to aerospace structures. Many products exist

as a result of adhesive bonding or sealing. If the total value added to our economy by the

relatively small amount of adhesives and sealants that are used is to be determined, the

result would be staggering as adhesives and sealants all around us, with applications

extending back to at least biblical times, and with many examples of outstanding

adhesion in nature (e.g., barnacles and ice on roads).

The adhesives and sealants industry is bolstered by thousands of years of trial

and error. This long history can be coupled with significant additions to the fundamental

supporting sciences and with the development of advanced materials and processes. The

study of adhesives and sealants and the sciences surrounding their application has never

been more important. Adhesives and sealants are often made of similar materials, and

they are sometimes used in similar applications. These materials have comparable

processing requirements and failure mechanisms, and the fundamentals of how they

work are similar. However, the different specifications and test methods apply to

adhesives and sealants, and most often they are designed to perform different functions.

Their definitions show their different functions (Petrie, 2000).

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Adhesive—a substance capable of holding at least two surfaces together in a strong and

permanent manner.

Sealant—a substance capable of attaching to at least two surfaces, thereby, filling the

space between them to provide a barrier or protective coating.

Adhesives and sealants are often considered together because they both adhere

and seal. Both must be resistant to their operating environments; and their properties are

highly dependent on how they are applied and processed. Adhesives and sealants also

share several common characteristics (Elias 2009; Harshorn 2009; Ebnesajjad 2011).

They must behave as a liquid, at some time in the course of bond formation, in

order to flow over and wet (make intimate contact with) the adherends.

They form surface attachment through adhesion (the development of

intermolecular forces).

They must harden to carry sometimes continuous, sometimes variable load

throughout their lives.

They transfer and distribute load among the components in an assembly.

They must fill gaps, cavities, and spaces.

They must work with other components of the assembly to provide a durable

product.

Adhesives are chosen for their holding and bonding power. They are generally

materials having high shear and tensile strength. Structural adhesive is a term generally

used to define an adhesive whose strength is critical to the success of the assembly. This

term is usually reserved to describe adhesives with high shear strength (in excess of

1,000 pounds per square inch or psi) and good environmental resistance. Examples of

structural adhesives are epoxy, thermosetting acrylic, and urethane systems.

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Structural adhesives are usually expected to last the life of the product to which

they are applied and while, non structural adhesives are adhesives with much lower

strength and permanence. They are generally used for temporary fastening or to bond

weak substrates. Examples of non-structural adhesives are pressure sensitive films,

wood glue, elastomers, and sealants. Sealants are generally chosen for their ability to fill

gaps, resist relative movement of the substrates, and exclude or contain another

material.

They are generally lower in strength than adhesives, but have better flexibility.

Common sealants include urethanes, silicones, and acrylic systems. Both adhesives and

sealants function primarily by the property of adhesion. Adhesion is the attraction of

two different substances resulting from intermolecular forces between the substances.

This is distinctly different from cohesion, which involves only the intermolecular

attractive forces within a single substance. The intermolecular forces acting in both

adhesion and cohesion are primarily van der Waals force. For better understanding on

the difference between adhesion and cohesion, consider the failed joints illustrated in

Figure 1.1. Joints fail either adhesively or cohesively or by some combination of the

two.

Adhesive failure is an interfacial bond failure between the adhesive and the

adherent. Cohesive failure could exist within either the adhesive material or the

adherent. Cohesive failure of the adhesive occurs when stress fracture within the

adhesive material allows a layer of adhesive to remain on both substrates (i.e. the

attachment of the adhesive to the substrate is stronger than the internal strength of the

adhesive itself, and the adhesive fails within its bulk). When the adherent fails before

the adhesive and the joint area remains intact, it is known as a cohesive failure of the

adherent.

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Figure 1.1: Examples of cohesive and adhesive failure

1.2 Adhesive classifications

Adhesives are classified by many methods, and there can be many hierarchical

levels to these classification schemes. The broadest classification scheme is to

categorize an adhesive as being manufactured from materials that are either synthetic or

naturally occurring. Synthetic adhesives are manufactured from man-made materials

such as polymers. Natural adhesives are manufactured from naturally occurring

materials such as animal or agricultural by-products. Many adhesives are made from

organic polymers. There are also adhesive systems with inorganic origin. The oldest

polymers used for adhesives were of natural origin. Often naturally occurring adhesives

are thought to be inferior to synthetic polymers because of their lower strength and

limited freedom in processing. However, in many applications, such as bonding of paper

and wood where the emphasis may be on the adhesive being biodegradable or

repulpable, naturally occurring adhesives find a strong market (Harshorn 2009). Modern

epoxies, urethanes, acrylics, and other adhesive systems that are used in demanding

structural applications are made from synthetic polymers.

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The classification of adhesives into synthetic and naturally occurring categories

is usually far too broad for many practical purposes. The industry has settled on several

common methods of classifying adhesives that satisfy most purposes. These

classifications are by:

Function

Chemical composition

Mode of application or reaction

Physical form

Cost

End-use

All of these classifications and distinctions overlap to some degree.

1.2.1 Function

The functional classification defines adhesives as being either structural or

nonstructural. Structural adhesives are materials of high strength and permanence.

Generally, structural adhesives are defined as those having shear strengths in excess of

1000 psi and resistance to most common operating environments. Their primary

function is to hold structures together and be capable of resisting high loads without

deformation. Structural adhesives are generally presumed to survive the life of the

application. Conversely, nonstructural adhesives are not required to support substantial

loads, but they merely hold lightweight materials in place. Nonstructural adhesives

creep under moderate load and are often degraded by long term environmental

exposures. They are often used for temporary or short term fastening. Nonstructural

adhesives are sometimes referred to as holding adhesives. Certain pressure sensitive

adhesives, hot melt, and water emulsion adhesives are examples of nonstructural

adhesives because they have moderately low shear strength, high creep, and poor

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resistance to temperature and chemicals. However, at times these materials could

possibly be used in long term applications, depending on the severity of the application.

Nonstructural adhesives are sometimes used with other types of fasteners including

mechanical fasteners. In these applications, the adhesive bond is considered a secondary

fastener. The use of a nonstructural adhesive in concert with mechanical fasteners may

allow one to reduce the number of mechanical fasteners that would normally be used

and also provide additional value in the assembly such as vibration damping, sealing, or

insulation.

1.2.2 Chemical composition

The classification of adhesives by chemical composition describes adhesives in

the broadest sense as being thermosetting, thermoplastic, elastomeric, or alloys

(hybrids) of these. These classifications are described in Table 1.1. Usually, the

chemical composition is divided further into major chemical types or families within

each group, such as epoxy, urethane, neoprene, and cyanoacrylate.

1.2.2.1 Thermosetting adhesives.

Thermosetting adhesives are materials that cannot be heated and softened

repeatedly after their initial cure. Once cured and crosslinked, the bond can be softened

somewhat by heat, but it cannot be remelted or restored to the flowable state that existed

before curing. Thermoset materials form infusible and insoluble materials. These

adhesives generally degrade and weaken upon heating at high enough temperatures

because of oxidation or molecular chain scission. Thermosetting adhesive systems cure

by an irreversible chemical reaction at room or elevated temperatures, depending on the

type of adhesive. This chemical reaction is often referred to as crosslinking.

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The cross-linking that occurs in the curing reaction is brought about by the

linking of two linear polymers, resulting in a three dimensional rigidized chemical

structure. Crosslinking usually occurs by chemical reaction. With certain room

temperature curing adhesives, it is the internal heat of reaction generated by the curing

mechanism, called the exotherm, which actually provides the energy required to

completely cure the polymeric material. Substantial pressure may also be required with

some thermosetting adhesives, yet others are capable of providing strong bonds with

only moderate contact pressure. Thermosetting adhesives are sometimes provided in a

solvent medium to facilitate application by brush or spray. However, they are also

commonly available as solvent less liquids, pastes, and solid shapes. Epoxy and

urethane adhesives are examples of common adhesives that are in the thermoset

chemical family. Thermosetting adhesives may be sold as multiple or single part

systems. Multiple part systems have their reactive components separated.

They are weighted-out and mixed together at the time of application. Multiple

part adhesives generally have longer shelf lives, and they are usually cured at room

temperature or more rapidly at elevated temperatures. Once the adhesive components

are mixed, the working life or gel time is limited. Single part systems have all the

reactive components in a single premixed product. Generally, the single part adhesives

require elevated temperature cure. They have a limited shelf life, often requiring

refrigeration. Single part thermosetting systems are also available that cure at room

temperature by chemical reaction with the moisture in the air, by exposure to radiation

(visible, UV, electron beam, etc.), or by catalytic reaction with a substrate surface.

Because molecules of thermosetting resins are densely crosslinked, their resistance to

heat and solvents is good, and they show little elastic deformation under load at elevated

temperatures. As a result, most structural adhesives tend to be formulated with

polymeric resins having a thermosetting molecular structure.

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Table 1.1: Adhesives classified by chemical composition

Classification

Thermoplastic Thermosetting Elastomeric Alloys

Types within

group

Cellulose acetate,

cellulose acetate

butyrate,

cellulose nitrate,

polyvinyl

acetate,

polyvinyl

alcohol,

polyamide,

acrylic phenoxy

Cyanoacrylate,

polyester, urea

formaldehyde,

melamine

formaldehyde, epoxy,

polyimide,

polybenzimidazole,

acrylic, acrylate acid

diester

NR, reclaimed

rubber,butyl,

nitrile,

polyisobutylene,

styrene-butadiene,

polyurethane,

polysulfide,

silicone, neoprene

Epoxy-phenolic,

epoxy-

polysulfide,

epoxy-nylon,

nitrilephenolic,

neoprene-

phenolic, vinyl –

phenolic

Most used

form

Liquid, some dry

film

Liquids, but all forms

common

Liquids, some

film

Liquids, paste,

film

Common

further

classifications

By vehicle (most

are solvent

dispersions or

water emulsions)

By cure requirements

(heat and /or pressure

most common but

some are catalyst

types)

By cure

requirements (all

are common),also

by vehicle (most

are solvent

dispersions or

water emulsions)

By cure

requirements

(usually heat and

pressure except

some epoxy

types),by vehicle

(most are solvent

dispersions or

100% solids),and

by type of

adherents

Bond

characteristic

Good to 150-200

F, poor creep

strength, fair peel

strength

Good to 200 -500 F,

good creep strength,

fair peel strength

Good to 150-

400F, never melt

completely, low

strength, high

flexibility

Balanced

combination of

properties of

other chemical

groups depending

on formulation;

generally higher

strength over

wide temperature

range

Major type of

use

Unstressed

joints, design

with caps,

overlaps,

stiffeners

Stressed joints at

slightly elevated

temp

Unstressed joints

on lightweight

material, joints in

flexure

Where highest

and strictest end

service conditions

must be met, as

military uses.

Materials most

commonly

bonded

Formulation

range covers all

materials, but

emphasis on

nonmetallic –

esp. wood,

leather, cork,

paper, etc

For structural uses of

most materials

Few used ‗straight

for rubber, fabric,

foil, paper,

leather, plastic

film; also as tapes.

Most modified

with synthetic

resins

Ceramics, glass,

thermosetting

plastic; nature of

adherents, not as

vital as design or

end service

conditions

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1.2.2.2 Thermoplastic adhesives.

Thermoplastic adhesives differ from thermosets in that they do not cure or set

under heat. Thermoplastics are originally solid polymers merely soften or melt when

heated. Their molecular structure is either linear or branched. Since thermoplastic

molecules do not cure into a crosslinked structure, they can be melted with application

of heat and then applied to a substrate. Thermal aging, brought about by repeated

exposure to the high temperatures required for melting, causes eventual degradation of

the material through oxidation, and this limits the number of reheat cycles. Once applied

to the substrate, the parts are joined and the adhesive hardens by cooling.

Hot-melt adhesives, commonly used in packaging, are examples of a solid

thermoplastic material that is applied in a molten state. Adhesion develops as the melt

solidifies during cooling. Thermoplastics can also be dissolved in solvent to produce a

flowable solution and then reharden on evaporation of the solvent. Thermoplastic resins

can also be dispersed in water as latex or emulsions. These products harden on

evaporation of the water. Wood glues, a common household item, are thermoplastic

resins that are dispersed in water as either a latex or emulsion. They harden by

evaporation of the water and coalescence of the resin into a film form.

Thermoplastic adhesives are also preapplied to a substrate so that they can be

activated at an appropriate time. Some thermoplastic adhesives make good use of this

characteristic and are marketed as heat, solvent, or moisture activated adhesives. The

best known example of a moisture activated adhesive is the mailing envelope adhesive

that is activated by moisture to become tacky and somewhat flowable. When the

moisture evaporates, the bond sets. Thermoplastic adhesives have a more limited

operating temperature range than thermosetting types. Although certain thermoplastics

may provide excellent tensile shear strength at relatively moderate temperatures, these

materials are not crosslinked and will tend to creep under load at lower temperatures.

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This creep or long term deformation under load can occur at room temperature

or even at lower temperatures depending on the adhesive. Long term creep is often the

characteristic that prevents these adhesives from being used in structural applications.

Thermoplastic adhesives also do not have general resistance to solvents or chemicals as

do the thermosetting adhesives.

1.2.2.3 Elastomeric adhesives

Because elastomeric adhesives have unique rheological characteristics, they are

given their own classification. Elastomeric adhesives are based on synthetic or naturally

occurring elastomeric polymers having great toughness and elongation. These adhesives

are made from polymeric resins that are capable of high degrees of extension and

compression. They return rapidly to their initial dimensions and shape after the load is

removed. As a result, elastomeric adhesives have great energy absorbing characteristics

and offer high strength in joint designs having nonuniform loading. Elastomeric

adhesives may be either thermosetting or thermoplastic. The thermosetting types can be

used in certain structural applications.

Elastomeric adhesives may be supplied as solvent solutions, water dispersions,

pressure-sensitive tapes, and single or multiple part solventless liquids or pastes. The

form and curing requirements vary with the type of elastomeric resin used in the

adhesive formulation. Elastomeric adhesives can be formulated for a wide variety of

applications. Because elastomers are highly viscoelastic materials, they are

characterized by a high degree of elongation, low modulus, and high toughness. This

provides adhesives with high peel strength and a high degree of flexibility to bond to

substrates with different expansion coefficients. Elastomers are also commonly used in

adhesive formulation for sealants, vibration dampers, and sound enclosures.

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1.2.2.4 Hybrid adhesives.

Adhesive hybrids are made by combining thermosetting, thermoplastic, or

elastomeric resins into a single adhesive formulation. Hybrids have been developed to

capitalize on the most useful properties of each component. Generally high temperature,

rigid resins are combined with flexible, tough elastomers or thermoplastics to provide

improved peel strength and energy absorption. However, early attempts at these

combinations usually resulted in an adhesive that was never better than its weakest

constituent. The good high temperature properties of the base resin were always

sacrificed by the addition of the flexibilizing additive. The earliest approach to combat

brittle failure was to develop adhesive formulations by blending a flexibilizing resin into

the body of another resin. Thus, nitrile-phenolic, epoxy-polysulfide, and other resin

blends provide resilience and toughness due to the elastomeric ingredients in the

formulation.

A limitation to these systems was that the elastomeric component usually

lowered the glass transition temperature and degraded at the elevated temperature and

lowered or raised chemical resistance that was characteristic of the more rigid resin.

These alloy blends are still in wide use today especially in the aerospace and

transportation adhesive markets. They are commonly available in solvent solutions and

as supported or unsupported film. Some single and two part liquid systems are also

available. More recently, advanced hybrid adhesive systems have been developed in an

attempt to improve peel strength and toughness of thermosetting resins without reducing

high temperature properties.

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1.2.3 Mode of reaction

Another distinction that can be made between adhesives is the manner in which

they react or solidify. There are several methods by which adhesives can solidify:

1.2.3.1 Chemical reaction

Most thermosetting adhesives crosslink and cure by two primary chemical

reactions. It also includes reaction with a hardener or reaction with an outside energy

source such as heat, radiation, surface catalyst, etc. There are many adhesives that cure

by chemical reaction and can be further subdivided into groups:

a. Two part systems

b. Single part, cured via catalyst or hardener

c. Moisture curing adhesives

d. Radiation (light, UV, electron beam, etc.) curing adhesives

e. Adhesives catalyzed by the substrate

f. Adhesives in solid form (tape, film, powder, etc.)

1.2.3.2 Solvent / Water loss

Solvent solutions and water based latexes and dispersions harden via evaporation

of their carrier material either solvent or water. The function of the carrier material is

simply to lower the viscosity of the adhesive so that it can be easily applied to the

substrate. Once applied, the water or solvent must be removed either by evaporation into

the air or by diffusion into a porous substrate. Thus, solvent and water based systems

often find use in applications with porous substrates such as wood, paper, leather, and

fabric. Once applied and dried, the adhesive can then provide a bond in a number of

ways. It could simply harden into cohesive resin mass, such as polyvinyl acetate or

wood glues.

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It could form a film that is then reactivated by solvent or water or by heat as in

the case of a laminating adhesive. It could also form a film that when dry has a high

degree of tack so as to be a pressure sensitive adhesive. There are primarily four types

of adhesives that harden by loss of solvent or water:

Contact adhesives:- The adhesive is applied to both substrates, solvent is

removed, and the substrates are mated under pressure so that the adhesive

coatings knit together.

Pressure sensitive adhesives:- The adhesive is applied to one or both substrates

or to a carrier (film, cloth, etc.). Once the solvent is removed, the adhesive has

aggressive and permanent tackiness. The substrates can be mated with very little

pressure.

Reactivatable adhesives:- The adhesive is applied to the substrate and the solvent

is evaporated. At this stage the substrate may be stored or transported with a dry

adhesive coating. At the time of joining, the coating is moistened with the

solvent and the adhesive becomes tacky with pressure sensitive characteristics.

Resinous solvent adhesives:- The adhesive is a resinous material dissolved in the

solvent. It is applied to porous substrates that are then joined. The solvent

evaporates leaving the resin to mechanically lock the substrates together.

1.2.3.3 Cooling from a melt

Adhesives can also harden by cooling from a melt condition. Hot melt adhesives

are the most common example of this type of adhesive. These are generally

thermoplastic adhesives that soften and melt when heated, and they harden on

subsequent cooling. The hot melt system must achieve a relatively low viscosity when

in the molten state to achieve wetting, and it must not cool too rapidly or it will not have

time to completely wet the roughness of the substrate surface.

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Once the hot melt adhesive is applied and in the molten condition, the substrates

must be joined immediately. The adhesive can be applied and then the adhesive coated

substrate is placed into storage for later activation. At a latest date, the coated substrates

could then be removed from storage, reheated to soften the adhesive, and joined under

slight pressure. When hardened, the hot melt adhesive can have various degrees of

tackiness depending on the formulation. A completely pressure sensitive adhesive could

result from a hot melt formulation. Some pressure sensitive tapes and films are

manufactured in this manner. This method eliminates the expense and environmental

hazards of using solvent simply to reduce the viscosity of a pressure sensitive resin for

application to a carrier.

1.2.4 Physical form

A widely used method of distinguishing between adhesives is by their physical

form. Table 1.2 showed adhesives classified by form. Adhesive systems are available in

a number of forms. The most common forms are:

Multiple part solventless (liquid or paste)

One part solventless (liquid or paste)

One part solution (liquid)

Solid (powder, tape, film, etc.)

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Table 1.2: Adhesive classified by form

T Type Remarks Advantages

Liquid

Most common form; practically

every formulation available.

Principally solvent-dispersed

Easy to apply. Viscosity often under

control of user. Major form for hand

application.

Paste Wide range of consistencies.

Limited formulations. Principally

100% solid modified epoxies

Lends itself to high-production

setups

because of less waiting time. High

shear and creep strength.

Powder Requires mixing or heating to

activate curing

Longer shelf life; mixed in quantities

needed

Mastic Applied with trowel

Void-filling, nonflowing

Film,

tape

Limited to flat surfaces, wide

range of curing ease

Quick and easy application. No waste

or run over, uniform thickness

Other Rods, supported tapes, precoated

copper for printed circuits, etc

Ease of application and cure for

particular use.

1.2.5 Cost

Cost is usually not used as a method of classifying adhesives; however, it is an

important factor in the selection of a specific adhesive and a factor in determining

whether adhesives should be used at all. Thus, cost becomes a means of classification

and selection, if not directly, at least indirectly. The raw materials cost or ‗‗first cost‘‘ of

an adhesive can vary significantly. Adhesive price is dependent on development costs

and volume requirements. Adhesives that have been specifically developed to be

resistant to adverse environments are usually more expensive than general purpose

adhesives. Adhesive prices range from penies a pound for inorganic and animal-based

systems to several hundred dollars per pound for some heat-resistant synthetic types.

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Adhesives in film or powder form require more processing than liquid or paste

types and are usually more expensive. When estimating the cost of using adhesives, one

must not only consider the price of the adhesive but also the cost of everything required

to obtain a reliable, complete joint. Therefore, the cost of fastening with adhesives must

also include the cost of labor, the cost of equipment, the time required to cure the

adhesive, and the economical loss due to rejects of defective joints. The following

parameters may be important in analyzing the real cost of an adhesive system:

Efficiency of coverage in relation to bonding area or number of components.

Ease of application and processing equipment needed (jigs, ovens, presses,

applicators, etc.)

Total processing time (for assembly, for preparation of adherents, for drying, for

curing, etc.)

Cost of labor for assembly and inspection of the bonded parts.

Waste of adhesive contributes to material costs and environmental costs for disposal.

Amount of rejected material as compared with other methods of joining.

1.2.6 End use

Adhesives may also be classified according to their end-use. Thus, metal

adhesives, wood adhesives, and vinyl adhesives refer to the substrates to which they will

bond. Similarly, acid-resistant adhesives, heat-resistant adhesives, and weatherable

adhesives indicate the environments for which each is best suited. Adhesives are also

often classified by the method in which they are applied. Depending on viscosity, liquid

adhesives can be considered to be sprayable, brushable, or trowelable. Heavily bodied

adhesive pastes and mastics are considered to be extricable; they are applied by syringe,

caulking gun, or pneumatic pumping equipment (Midgley 1990).

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1.3 Solvent and water -based adhesives

Solvent - based pressure sensitive adhesives have a broad material basis. They

can be formulated as tackified elastomers, acrylics, polyurethanes and other materials.

Their formulation can be either the manufacture of an adhesive (e.g. rubber resin PSA)

or the modification of an adhesive. Solvent based acrylics are recommended for durable

mounting tapes, outdoor pressure sensitive products, medical tapes and protective tapes.

They offer possibility of various cross linking with or without previous destruction of

base polymer. Thus, their adhesive and end use properties especially tack and shears are

easy controlled (Petrie, 2000).

In the first stage of development NR-resin-based formulations were used as

solvent based adhesives for tapes and label. Later, synthetic rubbers were introduced.

Cross-linking of solvent-based formulations has been used since the beginning of PSA

technology. Solvent – based cross linked formulations were developed for each product

class due to their relatively broad raw material basis and easy regulation of their cross

linking degree by the aid of raw materials and manufacturing condition. At first NR was

used and later synthetic elastomers were processed. Each requires special technology.

Formulation of NR includes different procedures to improve the adhesive,

cohesive, and aging characteristics. Tackification of NR is the classic way to obtain

PSAs. NR can be used together with polar elastomers and reactive resin to achieve

temperature and solvent resistance. The reaction is carried out by mastication of the

components. Synthetic rubber-based PSA formulations were developed as solvent –

based, water-based, and 100% solids. The best known synthetics elastomers used for

solvent-based PSAs are the styrene-diene copolymer, polyisobutene, acrylnitryl-diene

copolymers, silicones, polyurethane and etc (Robinson, 1979).

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Water–based adhesives are considered as replacements for solvent based

adhesives for the purpose of reducing volatile organic emissions in a manufacturing

operation. Water–based adhesives are usually emulsions of thermoplastic resins. The

properties of the emulsion are derived from the polymer employed as well as the system

used to emulsify the polymer in water. There are several additives necessary to stabilize

and protect the emulsion. Other additives are used to adjust tack, drying time, viscosity,

storage stability, etc. Like the solvent-based adhesives, the water carrier is evaporated

through the air or diffused into the porous substrate. When dry the resulting adhesive

can be either a brittle, hard resin or a flexible, tacky film depending on the adhesive

formulation.

Water–based adhesives are like solvent–based adhesives in that they are

formulated as contact, pressure sensitive, reactivatable, and resinous adhesive systems.

They are used much the same way as their solvent based counterparts. The most widely

used emulsion-based adhesive is the polyvinyl acetate-polyvinyl alcohol copolymer,

known as ‗‗white‘‘ glue or wood glue. This adhesive hardens to a relatively rigid solid

when the water diffuses through the substrate. Although there are several water–based

contact and pressure sensitive adhesives on the market, they are slower drying than

solvent-based adhesives, requiring about three times more heat to dry. Forced drying of

water-based adhesives costs energy and causes corrosion problems in the ovens that are

used. Water–based contact adhesives also have a lack of immediate bonding capability.

When cured, the water–based adhesives do not have the moisture resistance that solvent-

based contact adhesives have. For water–based PSAs, formulation is mostly a

modification of the base adhesive, but it is also focused on the adhesive and converting

properties.

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In this case, the modification of the base adhesive is more complex because of

its built-in chemical composition and its dispersed system–related chemical composition

(particles with composite structure).Therefore, formulation affects the dispersion

stability as well. For the dispersed system, formulation also depends on the storage time.

Dispersing quality affects the static shelf–life of the dispersion and its shear stability. In

some cases dispersing affects the adhesive properties of the PSA. For instance, when

water–based viscoelastic and viscous components are mixed (e.g. acrylic with

tackifiers), peel resistance increases with storage time of the mixture (up to limit).

Generally, in the formulation of water based adhesives, special additives are included

during synthesis and formulation (Elliott and Glass, 2000).

1.4 Pressure sensitive adhesives (PSAs)

Pressure sensitive adhesives (PSAs) are polymeric materials that exhibit

viscoelastic properties, maintain aggressive and permanent tack, and have enough

cohesive strength to be adhesively removed from a surface without leaving a noticeable

residue. PSAs require no more than finger pressure for adhesion and the adhesion force

is relatively insensitive to applied pressure. In addition, no chemical reaction or physical

change occurs in the PSA on bonding which primarily occurs through van der Waals

forces. The debonding mechanism involves cavitations at the interface and fibril

formation within the PSA. Pressure-sensitive adhesives tapes was first developed in

1845 by Dr. Horace Day, a surgeon, who devised a method of applying a natural rubber

adhesive to strips of cloth, thus producing a kind of surgical tape which he used in his

practice. PSAs are coated on a film like backing as a carrier and once the solvent

evaporates, the tape can be used. Usually, they are found in all types of packaging and

masking tapes. Numerous industrial and consumer applications utilize PSA technology.

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Self-adhesive labels, masking tape, sealing tape, duct tape, removable adhesive

notes, postage stamps, transdermal patches and wound dressings are some common

household products that use PSAs. The market watch for PSA industry shows that world

demand for PSA tape is expected to grow five percent annually through 2012. A total

output of 36 billion sq meters of PSA tape is estimated globally in 2012, and 3.2 million

metric tons of raw materials (adhesives, substrates and release liners) will be needed to

convert the finished product. The PSA base resin can consist of a variety of chemistries

including polyacrylate, polyvinylether, silicone, polyisoprene, and polyisobutylene.

Tackifiers and plasticizers are often added to fine tune the physical properties of the

adhesive. PSAs are generally coated onto a backing material to form a PSA tape. Cloth,

paper, cellophane, polypropylene and polyester are some of the more common backing

materials that support the thin adhesive layer.

PSAs can be formulated to conduct heat and electricity, insulate, resist microbial

and environmental attack, and minimize specific reactions to the adherent such as

corrosion or sensitivity to skin. PSAs operate in humid and dry air, vacuum and

atmospheric conditions, and a wide array of temperatures. Because of the widespread

use of PSAs, understanding the mechanical behavior of the PSA throughout the range of

operating temperatures and conditions is extremely important. The function of PSAs is

to ensure instantaneous adhesion upon application of a light pressure. Most applications

further require that they can be easily removed from the surface to which they were

applied through a light pulling force. Thus PSAs are characterized by a built-in capacity

to achieve this instantaneous adhesion to a surface without activation, such as a

treatment with solvents or heat, and also by having sufficient internal strength so that

the adhesive material will not break up before the bond between the adhesive material

and the surface ruptures (Wool, 2005 a ).

21

The bonding and the debonding of PSAs are energy-driven phenomena.

Pressure-sensitive adhesives must possess viscous properties in order to flow and to be

able to dissipate energy during the adhesive bonding process. However, the adhesive

must also be elastic (i.e., it must resist the tendency to flow) and, in addition, store bond

rupture energy in order to provide good peel and tack performance. Pressure-sensitive

adhesives should possess typical viscoelastic properties that allow them to respond

properly to both a bonding and a debonding step. For satisfactory performance in each

of these steps the material must respond to a deforming force in a prescribed manner.

Polymers employed as PSAs have to fulfill partially contradictory requirements; they

need to adhere to substrates, to display high shear strength and peel adhesion, and not

leave any residue on the substrate upon debonding. In order to meet all these

requirements, a compromise is needed.

When using PSAs there appears another difference from wet adhesives, namely

the adhesive does not change its physical state because film forming is inherent to

PSAs. Thus, PSAs used in self-adhesive laminates are adhesives which, through their

viscoelastic fluid state, can build up the joint without the need to change this flow state

during or after application. On the other hand, their fluid state allows controlled

debonding giving a temporary character to the bond. Because of the fluid character of

the bonded adhesive, the amount of adhesive (i.e., the dimensions of the adhesive layer)

is limited; the joint works as a thin-layer laminate or composite. Because of this special,

thin-layer structure of the composite, the solid state components of the laminate exert a

strong influence on the properties of the adhesive. Therefore, there exists a difference

between the measured properties of the pristine adhesive and of the adhesive enclosed

within the laminate. Adhesives, in general, and PSAs, in particular, have to build up a

continuous, soft (fluid), and tacky (rubbery) layer. The latter will adhere to the substrate.

22

On the other hand, the liquid adhesive layer of the PSAs working in the bond has

to offer a controlled bond resistance. This special behavior requires materials exhibiting

a viscoelastic character. The properties which are essential in characterizing the nature

of PSAs comprise of tack, peel adhesion, and shear. The first measures the adhesive‘s

ability to adhere quickly, the second its ability to resist removal through peeling, and the

third its ability to hold in position when shear forces are applied. PSAs are applied much

like contact adhesives and instantly provide a degree of tackiness. Unlike contact

adhesives, their tackiness is permanent and there is no optimal time range when the

substrates must be joined. PSAs are usually based on elastomer or thermoplastic solvent

solutions. They are coated on the substrate or on a film backing that is used as a carrier.

Pressure sensitive tape (e.g., office tape) is made in this manner. Once the solvent

evaporates, the tape is ready to be applied. So that it can be dispensed at the point of

assembly (Wool, 2013).

Most PSAs are made from formulations based on elastomer (natural, butyl,

nitrile and styrene butadiene thermoplastic), acrylate, or silicone resins. Table 1.3 shows

advantages and limitations of PSAs based on elastomer. PSAs are specifically

formulated for good flexibility, tack, and peel strength and can be applied from solvent

solutions, water dispersions, or hot melts. Thermoplastic elastomers are usually

employed in hot melt PSAs. Most PSAs are applied to plastic, paper, foil or fibrous

material as suitable backings. Pressure sensitive tapes are made with adhesive on one

side or on both sides (double faced tape). Double faced foam tapes are available to suit

applications where substrates have surface irregularities, contours, and significant gaps

in the joint area. Several very high bond strength adhesive tapes have been developed

for semi-structural applications. Use of foam carrier provides not only gap filler but also

helps to distribute the stress within the joint area, thus, improving the ultimate strength

of the joint.

23

These tapes offer bond strengths up to 10 times those of conventional PSA while

maintaining enough cohesive strength to support modest loads for long periods of time.

PSAs provide a relatively low strength bond upon the brief application of slight pressure

(usually by hand). They may be applied to any clean, dry surface. Since they are capable

of sustaining only light loads because of creep, they are not considered structural

adhesives. PSAs build a stronger bond over time. One should not test PSA immediately

after they are applied and the joint is made. Full strength may take several days to

develop and pressure sensitive tapes require relatively close mating substrates. The PSA

film thickness may be only several mils thick. If there are gaps larger than that between

the substrates, the adhesive will not bond because there is no applied pressure in the area

of greatest gap. Pressure sensitive tapes using foam (polyethylene or vinyl) carriers are

used where gap tolerances may be great. These adhesives are easily recognized in

automobile protective side strips, mounting tape for wall dispensers, etc (Benedek,

2004).

1.4.1 Rubber – based pressure sensitive adhesives (PSAs)

Rubber is composed of very large molecules with high molecular weight. As the

molecular weight increases, the density, melting point, and boiling point increase, with

the last property increasing to the point where the material decompose before it

evaporates. High molecular weight is necessary for strong, load-bearing rubbers.

Viscosity is roughly proportional to the molecular weight of the rubbers. To make a

rubbery polymer, it is necessary to minimize crystallization by breaking up the

structural regularity of the repeating –CH2- segments. There are two ways to prevent

regular alignment in rubbers (Martín-Martínez, 2002).

24

Table 1.3: Show the advantages and limitations of PSAs based on elastomer

Chemical

family

Rubbers Acrylates Silicones

Advantages

1) Good flexibility

2) High initial

adhesion(better than

acrylic)

3) Ease of

tackification (with

additives)

4) Lower cost

5) Good shear

strength

6) Good adherence

to low and high

energy surfaces

7) Suitable for

temporary or

permanent holding

1) Good UV resistance

2) Good hydrolysis

resistance(better than

rubber)

3) Excellent adhesion

buildup

4) Good solvent

resistance

5) Good temperature use

range (-45 to 121oC)

6) Easier to apply than

rubber

7) Good shear strength

and service life

1) Excellent

chemical and

solvent

resistance

2) Wide

temperature used

range(-73 to

260oC)

3) Good

oxidation

resistance

4) Good

adherence to low

and high energy

surfaces

Disadvantages

1 )Low tack and

adhesion (without

additives)

2) Poor aging,

subject to yellowing

3) Limited upper

service temperature

used and moderate

service life

1) Poor creep

resistance(compared to

rubber)

2) Fair initial adhesion

3) Moderate

cost(compare to rubber

and silicone)

1) Higher cost

compared than

rubbers and

acrylates

2) Lack of

aggressive

behavior

25

1) Add side groups such as methyl or chlorine. The methyl group (for instance in

ethylene-propylene rubber) prevents neighboring chains segments from aligning

perfectly.

2) Include unsaturation or carbon-carbon double bonds in the polymer chain. Diene

monomers have two carbon-carbon double bonds and polymerize in such a way that the

repeating segments are joined at extreme carbon atoms with the other double bond

remaining in each segment. These remaining carbon-carbon double bonds prevent

rotation, hindering the alignment of the molecular segments and disturbing

crystallization. Styrene-butadiene rubbers are noncrystallizing polymers containing

unsaturation

Both side groups and carbon-carbon double bonds can be incorporated into the

polymer structure to produce highly resilient rubbers. One typical example is

polyisoprene (PI) rubber. When temperature is lowered, rubber becomes stiff and brittle.

All rubbers eventually stiffen to a rigid, amorphous glass at the glass transition

temperature (Tg). This temperature also indicates the low temperature service limit of

the rubber. Tg values are dependent on the structure, degree of cross-linking

(vulcanization), and isomer composition of the rubber. The physical properties of the

rubbers are mainly determined by the molecular weight and the structure of the

repeating units.

By making branched chains rather than linear ones, low-viscosity polymers for

solution applications are obtained. By lightly cross – linking to form an insoluble or gel

polymer, better extruding polymer can be obtained. By broadening the molecular weight

distribution, polymer that mixes more easily can be obtained. Several elastomers can be

used in rubber–based PSAs, mainly NR, butyl rubber (BR), polyisobutylenes (PIB), and

styrene-butadiene rubber. Typical properties of these rubbers are given in Table 1.4.

26

The elastomers provide the backbone of the adhesive, so the main performance

of the adhesive is provided by the rubber properties. However, several specific

properties for application are imparted by adding other ingredients to the formulations.

Table 1.4: Some properties of elastomers used in rubber - based PSAs

Rubber-based adhesives, also called elastomeric adhesives are widely used in

industrial and household applications. In fact, about one-third of the adhesives used in

the world are made from natural or synthetic rubbers. Some of the elastomeric adhesive

systems demonstrating industrial importance in recent years are pressure-sensitive tapes

and labels, construction adhesives, contact adhesives, hot-melt packaging and

bookbinding adhesives, and high strength structural applications for aircraft,

automotive, and construction.

Some rubber – based adhesives need vulcanization to produce adequate ultimate

strength and the adhesion is mainly due to chemical interaction at the interface. Other

rubber based adhesives (contact adhesives) do not necessarily need vulcanization, but

require adequate compounding to produce the adhesives joints, mainly with porous

substrate. One of the most common classes of rubber-based adhesives is the contact

adhesives. In broad sense, it can be considered as PSAs, because tackiness is a key

properties and pressure must be applied before joining.

Elastomer Density(g/cm3) Tg(

oC) Max. Service T(

oC)

Natural rubber 0.91 -75 70

Butyl rubber 0.92 -65 100

Styrene-butadiene rubber 0.93 -55 70

27

These adhesives are bound to themselves by a diffusion process in which the

adhesive is applied to both surfaces to be joined. To achieve the optimum diffusion of

the polymer chains, two requirements are necessary: (i) high wettability of the adhesive

by the smooth or rough substrate surfaces and (ii) adequate viscosity of the adhesive to

penetrate into voids and roughness of the substrate surfaces. Both requirements can be

easily achieved by liquids adhesives. The dry adhesive films on the two substrates to be

joined must be placed in contact to develop adequate auto adhesion. For example,

diffusion of polymer rubber chains must be achieved across the interface between the

two films to produce intimate adhesion at the molecular level. The application of

pressure or temperature for a given time allows the desired level of intimate contact

between the two adhesives film surface. The rheological and mechanical properties of

rubber-based adhesives will determine the degree of intimacy at the interface. Those

properties can be optimized by selecting the adequate rubber grade, the nature and

amount of tackifier, and the amount of filler, among other factors (Benedek, 2004).

1.4.2 Specific features of rubber–based PSAs

The chemical nature and molecular weight of the rubber will greatly determined

the properties of the PSAs. However, some common characteristic can be found in most

rubber-based PSAs as shown below:

1) Broad range of substrates for assembly. Rubber PSAs can be join to several

substrates in a temporary or permanent way. Although for many applications curing is

not necessary, to provide high strength and mainly heat and chemical resistance,

vulcanization is mandatory.

2.) Flexibility. The resilience of rubber helps to absorb the stress applied to the joints.

Therefore, these adhesives properly resist impact, shear, elongation, vibration, and peel

stresses.

28

3) High peel strength. The intrinsic properties of rubber (high ability to produce high

elongation under stress) impart adequate strength to the joint under peeling forces.

However, rubber-like polymers demonstrate poor resistance to shear stresses.

4) Versatility of formulation. Several types of elastomers can be used in elastomeric

adhesives. For each family of rubber, several grades and chemical modifications can be

achieved to impart specific properties to the joints. Furthermore, specific rubber-based

PSAs formulation for particular end uses can be easily achieved by adding several

ingredients. However, the basic properties of the formulations are provided by the

rubber nature.

5) High green strength. This is one of the most important properties of rubber-based

PSAs. The green (immediate) strength can be defined as the ability to hold two surfaces

together upon first contact and before the adhesive develops its ultimate bonding

properties when fully cured. Also, the green strength is the intrinsic capacity of

adhesives to strongly adhere to the substrate immediately after application.

The green strength can be modified by changing the solvent composition (for

solvent-borne adhesives) or by incorporating ingredients into the formulations

(tackifier). Green strength is essential in PSAs and in some polychloroprene rubber-

phenolic resin blends. Rubber-based adhesives develop strength faster than most other

polymeric types. Figure 1.2 illustrates the differences in the development of peel

resistance for several rubber polymers (without additional additives, except an

antioxidant). NR and styrene-butadiene rubber provide higher initial peel strength

values. As time after joint formation progresses, polychloroprene and nitrile rubber

develop much higher peel resistance, particularly after a few hours (Benedek, 2004).

29

Figure 1.2: Bond strength development for different rubbers (Benedek, 2009)

1.5 Natural rubber (NR)

Natural Rubber (NR) can be obtained from the sap of a number of plants and

trees. The most common source is the Hevea Brasiliensis tree. It‘s first used as adhesive

was established in a patent dated 1891. As rubber became an important part of the

industrial revolution, the rubber adhesives market grew in importance. To comply with

the increasing demand for NR materials, plantation of H. brasiliensis trees were

established in Southeast Asia in the early 20th

century, mainly to meet the demand of the

automobile industry. NR is harvested as latex by tapping the trees in a similar manner as

for maple syrup. Trees latex contains about 35wt% rubber solid, as well as small

quantities of carbohydrates, resins, mineral salts, and fatty acids. Ammonia should be

immediately added to the latex to avoid coagulation by these other ingredients and to

prevent bacterial degradation. After collection, the latex can be concentrated to 60-70%

solid if the latex product is required for end use. Otherwise, latex is coagulated, washed,

dried and pressed into bales for use as dry rubber (Autenrieth, 1990).

30

NR latex grades are described by the method of concentration used. Evaporation,

creaming and centrifuging are the most common methods used in the industry.

(a) Evaporated latex is produced by heating at a reduced pressure, and the ammonia is

replaced by potassium hydroxide containing a small amount of soap to assist

stabilization. It has a solid content of about 73%. This NR latex grade shows improved

resistance to ageing and is adequate when high level of fillers have to be added during

formulation.

(b) Creamed latex is obtained by adding fatty acid soap and a creaming agent (e.g. an

alginate) in a tank; separation of the creamed layer from the serum must be completed.

The solids content in the creamed rubber is 66-69%. This grade of NR is bought after by

adhesive manufacturers.

(c) Centrifuged latex is the most important and common type of NR (about 95% of latex

production). Water-soluble non-rubber components are removed by centrifuging. The

solids content ranges from 50 to 67%. This NR latex grade is preserved by adding 0.7%

ammonia.

In solid form, the natural rubber is graded according to the content of dirt

remaining from the precipitation of latex at the plantation. Eight basic NR types have

been traditionally recognized internationally. Only the so-called fibbed smoked sheets

and the pale crepes are normally used for adhesives. The predominant grade system, the

Standard Malaysian Rubber system, has been used since 1965. There are some aspects

in the raw dry NR grades for adhesive manufacturing to be considered. NR tends to

suffer oxidative degradation catalyzed by metals (mainly copper). The susceptibility of

NR to oxidation can be measured using the plasticity retention index. The better grade

rubbers have a higher plasticity retention index .On the other hand, an increase in

viscosity and gel content of NR latex can be produced during storage (storage

hardening).

31

The presence of aldehyde groups on the rubber chain may produce cross-linking

reactions, which are responsible for the formation of gel. Addition of small amounts of

hydroxylamine to the latex before coagulation helps to prevent storage hardening. For

use in solution adhesives, NR must be masticated sufficiently to break down gel and

reduce viscosity for dissolution in suitable solvents. When low gel content is achieved,

viscosity-stabilized rubbers dissolve without the need for mastication. It is usual to form

the rubber into a thin layer to present a large surface to the solvent for rapid swelling.

This can be easily achieved by using particulate NR forms which are produced by

mechanical grinding of dry rubber. To avoid agglomeration of dry rubber particles

during storage and manipulation, an anti-tack agent (calcium stearate for example) was

added.

The chemical composition of NR mainly corresponds to cis-1, 4 polyisoprene

(Figure 1.3). Natural latex is polydisperse (size of individual particles may vary from

0.01 to 5 μm). However, synthetic latex has a relatively narrow particle size, and

therefore the viscosity at a given rubber content is higher in synthetic rubber

(polyisoprene) solutions. The average molecular weight is typically about 1 million

g/mol, although it depends on the gel content. The main characteristics of NR latex are

as follows: high gel content, high molecular weight, high cohesive strength, high self

adhesion and high solid content. The increasing demand on NR for several applications

and the increased production cost (labor, transportation, relatively low added value of

NR in the market) allows the manufacture of synthetic polyisoprene (PI) as an

alternative to NR.

Although PI should replace NR in adhesive formulations, there are some

differences in molecular weight and gel content and in the content in the trans isomer.

Therefore, the gel free nature of PI gives solubility in organic solvent without

mastication, but relatively poorer tack and green strength are obtained compared to NR.

32

With respect to the vulcanization adhesives, formulations containing PI tend to

cure more slowly and need the addition of higher amounts of tackifier than in NR

formulations. Due to that, NR still remains the major elastomer in the manufacturing of

PSAs. Therefore, in this study, NR was chosen in the formulation of PSA (Vaysse et al.

2012).

Figure 1.3: Chemical structure of the cis 1,4-polyisoprene

1.5.1 NR – based PSAs

NR–based PSAs were the first self –adhesive products used for the manufacture

of pressure sensitive products. Although NR is self adhesive, most elastomers must be

transformed into viscoelastomers to display usable pressure sensitivity (i.e., they must

be tackified). Most often, NR adhesives can be precoated onto fabric, paper, or film to

provide pressure sensitive tapes. NR based adhesives can be divided into two types, wet

bonding and dry bonding.

Wet bonding adhesives are applied on substrates in a fluid state, and the bond is

formed by drying. The dry bonding NR adhesives are pressure sensitive, and their

bonding does not need a supplemental liquid medium because it is allowed by their

special viscoelasticity. Adhesives made from the various forms of NR exhibit similar

characteristics, although some properties are altered by the addition of curatives. These

characteristic include the following:

---(CH2 CH2)n ---

C =C

H3C H

33

1. Excellent tack

2. Very good water and moisture resistance

3. High flexibility

4. Brittleness with age. Degradative oxidation can be produced, even after

vulcanization, due to oxygen and ozone attack to the double carbon-carbon bonds.

Adequate antioxidants must be added if aging is a key factor in performance.

5. Poor resistance to organic solvent and oils. It can be partially reduced in

vulcanized systems.

6. Low to moderate cost

7. A wide range of substrates can be bonded. The inherent tackiness of NR

enables it to coat most nonpolar substrates (mainly plastics and rubbers).

8. Good electrical and thermal insulators.

NR adhesives perform adequately under peeling stresses. The peel strength can

vary from a few N/m in PSA formulations to substrate tear in vulcanized compounds.

NR- PSAs with high tackifier content can be used as commercial tapes and surgical

plasters. These PSAs require the elimination of the gel fraction and a reduction in

molecular weight to facilitate solution. Rubber based solution PSAs are recommended

for label, protective films and tapes. Latex compounds have been typically used in

paper, textiles and construction. One of the popular applications is self sealing

envelopes. This application is based on the fact that when NR dries, some soluble non

rubber compounds migrates to the surfaces by water transport, leaving a thin film when

drying is completed. This film reduces the surface tack on the rubber and when presses

against a similar film, the non rubber layer is displaced, allowing the two rubber

surfaces to create bond.

34

It should be stressed that NR has excellent formulability in which it can be

depolymerized, detackified, re-polymerized by cross linking, and tackified with higher

polymers, tackifiers or plasticizers (Benedek and Feldstein, 2009).

1.6 Alkyd resin

Alkyd resins are polyfunctional oil-modified polyesters, synthesized by the

reaction of a polybasic acid together with a mono-functional acid. Kienle was the first

person to introduce the term ‗Alkyd‘ in 1927, and is derived from ‗al‘ of alcohol and

‗cid‘ of acid, where later ‗Kyd‘ was employed to give the desired phonics. Since alkyd

resins were first introduced some thirty-five years ago, they have enjoyed a consistent

annual growth, with current production now running well over one-half billion pounds.

Today alkyds outrank all other synthetic coating resins in importance, accounting for

approximately half of all resins used by the paint industry, which approaches a size of

two billion dollars annually in the United States. The alkyd reaction is concerned to be

the most versatile resin-forming reaction known. No other resin lends itself to greater

internal variation or to more useful modification by physical or chemical blending with

other polymers (Hofland, 2012).

Polymeric ester compounds were first synthesized in 1847 by Berzelius. During

1912-1915, the first alkyd resin sold commercially was marketed as ―Glyptals‖ by

General Electric Co. Alkyds resins are tough resinous products formed by reacting

polybasic organic acids with polyhydric alcohols. Broadly speaking, this type of

esterification reaction produces compounds of the general class of polyesters. The key

feature that distinguishes alkyds from other polyesters is the presence of monoacid

(commonly fatty acid) as a major part of its composition. Theoretically, any polyacid or

polyol should lend itself to the manufacture of alkyds.

35

However, from the standpoints of processing, paint performance, and pricing,

only a relatively few raw materials have found commercial acceptance for trade sales

and industrial applications. An alkyd is classed as a polymer (a huge molecule) formed

by chemical synthesis from many smaller molecules (Mańczyk, 2002).The process

whereby small unattached molecules are joined together by chemical reaction to form a

tight network of interconnected molecules is called polymerization. Since the alkyd

reaction usually releases a simple by-products molecule (commonly water) during the

molecular tie-up, the process can be thought of as a condensing action. For this reason,

the preparation of an alkyd is referred to as a condensation reaction and the end alkyd

product as a condensation polymer. To affect a chemical junction between two

molecules, it is necessary that they be mutually chemically reactive.

The reaction between a carboxyl group (—COOH) and a hydroxyl group (—

OH) is termed esterification. This reaction is basic to alkyd preparation. If there is only

one reactive site on each molecular species, i.e. if they are both monofunctional, it is

apparent that polymerization cannot occur, for even at 100 per cent reaction, no more

than double molecules can ever be formed. Let A and B be reactive sites on two

monofunctional molecular species (Patton, 1962). In fact, if either species of molecule

has but one reaction site (the other may have two or more sites), true polymerization

again fails to occur, for the most that can be expected is a saturation of the

polyfunctional molecule with attachments of the monofunctional molecule and this is

the maximum size of built-up molecule that can ever form under these conditions. It is

true that the resulting polyfunctional molecule may be quite large, but it can never

progress to the stage of infinite interconnections which is the hallmark of true polymeric

structures.

36

However, if neither of the two reacting molecular species is mono-functional

(they may be di–, tri–, tetra–, or of still higher polyfunctionalities), then infinite

networks of tied-together molecules become possible, giving polymers of huge

dimensions. This concept of functionality underlies the design of alkyd polymers

suitable for vehicles paint. At the very outset, it must be pointed out that the alkyd

chemist is constantly faced with a dilemma in formulating alkyd resins (Silvestre and

Gandini, 2008).

For an alkyd to be outstanding in performance, it must be processed to as high a

molecular weight as possible. At the same time, the molecular weight must not be

allowed to become excessive, or the alkyd will get out of control during processing

(convert to an intractable gel) or exhibit instability on shelf storage. Hence the alkyd

formulator must at all times avoid the design of either unduly small or unduly large

polymers. In general, an alkyd is formulated to a point just short of gelation at 100 per

cent reaction (Boruah et al. 2012).

1.6.1 Alkyd raw materials

1.6.1.1 Vegetable oils and resins

During the past one hundred years, the industry has employed large quantities of

fats and oils. The decline in fats and oils has largely been a result of the substitution of

petroleum products for the vegetable oils. Alkyd resin contains a large percentage of

fatty acids and has been the predominate binder used in the trade-sales and industry for

over thirty-five years (Güner et al. 2006). Petroleum based product usage increased

dramatically over the past decade, but it is noted that alkyd resins are still employed in

large quantities. A large percentage of the trade sales market has switched to water-

borne latex coatings based on petroleum derived materials.

37

The major reason for this is consumer convenience in clean-up, short dry times,

and low odor (Belgacem and Gandini, 2008). As coatings based on these products

further penetrate the existing market, the usage of vegetable oil-based coatings will

continue to decline. Vegetables oils are the modifier in an alkyd formulation. Examples

are palm oil, palm kernel oil, soybean oil, olive oil, colza oil, sesame oil, wood oil,

castor oil, linseed oil. Meanwhile, examples for animal oils are fish oil, whale oil, beef

oil, mutton oil and hoof oil (Silvestre and Gandini, 2008).

Natural resins such as rosin, amber, shellac and the synthetic ones such as

phenol resin, carbon resin and melamine resin also have been used in the alkyd

formulation. In addition, the naturally occurring oils consist of triglycerides, tri-esters of

glycerol and fatty acids (Abraham and Höfer 2012). The demand for fatty acids in paints

and coatings is expected to rise 2.5 percent per annum to 390 million pounds in 2002,

according to Fatty Acids Market Watch. Fatty acids, either in free form or in the form of

derivatives, are suited to a wide range of applications due to their versatility,

functionality, biodegradability and derivation from renewable resources. Some authors

classify oil as drying, semidrying and non-drying. This classification is based on the on

their iodine value which corresponds to degree of unsaturation in the oils (Wool, 2005).

1.6.1.2 Polyhydric alcohol

A polyhydric alcohol is an alcohol with more than one hydroxyl group (-OH).

Table 1.6 lists examples of common polyhydric alcohols used to synthesized alkyds.

Among these, glycerol is the most widely used polyol because it is present in naturally

occurring oils from which alkyds are generally prepared. Besides that, glycerol is

preferable due to its trifunctionality. This characteristic enables glycerol to add more

branching in the alkyd macromolecule. Therefore, would provide three dimensional

polymeric networks. Hence, results in a high molecular weight polymer produced.

38

Glycerol had two primary (-CH2OH) and one secondary (-CHOH) hydroxyl

groups. All the three hydroxyl groups react with fatty acids at 180-280oC. However, the

primary group is about ten times more reactive than a secondary one with dibasic acid

such as phthalic. Meanwhile, difunctional compounds, such as ethylene glycol, are

considered as chain extenders. They are mainly used to formulate unmodified resins

which are thermoplastic that are linear and cannot be crosslinked except in the case of

unsaturated polyesters.

Ethylene glycol is frequently used in the preparation of medium and short oil

alkyds to reduce hydroxyl number. However, the volatility of ethylene glycol posts a

problem in the production of polyester polyol. Generally, polyols are selected on the

basic of cost, rate of esterification, stability during high temperature processing, ease of

separation from water during processing, effect on Tg and others. Pentaerythritol is

polyvalent alcohol which is made by condensing formaldehyde and acetaldehyde in the

presence of alkali and water. It has four primary reactive hydroxyl groups. Because the

four hydroxyl groups are equivalent in reactivity, it would behave more uniformly than

glycerol when esterifies.

1.6.1.3 Polybasic acid

A polybasic acid is an acid containing at least two hydrogen bonding atoms

capable of substituting metal in a molecule, which means an acid having at least two

basicity. Examples of saturated dibasic acids are succinic acid, adipic acid, azelaic acid

and sebacic acid and for unsaturated dibasic acids, are terephthalic acid, maleic acid,

fumaric acid and isothalic acid. Meanwhile, anhydrides of saturated dibasic acids are

phthalic anhydride, maleic anhydride, glutaric anhydride and succinic anhydride.

Usually aromatic dibasic acids such as phthalic anhydride (PA) are used to prepare

polyester polyols/alkyds.

39

PA has the advantages that the first esterification reactions occur rapidly by

opening the anhydride ring and the amount of water evolved is lower. Thus, it will

reduce the reaction time. The shorter the time at lower reaction temperature would

reduce the side reactions. Besides that, PA is obtainable at low cost and could form

hydrogen bonding among molecules (Patton, 1962).Table 1.7list of common polybasic

acids used for preparation of alkyds.

40

Table 1.5: Some of the common polyhydric alcohols used for alkyd preparation

41

Table 1.6: List of common polybasic acids used for preparation of alkyd

42

1.6.2 Classification of alkyds

There are many ways of classifying alkyds. Generally, alkyd resins are classified

into different oil lengths (OL) based on the amount of oil (or fatty acid) in its

formulation and can be expressed in equations [1.1] and [1.2], respectively.

Oil length = Weight of oil × 100 [1.1]

Weight of alkyd – Water evolved

Oil length = 1.04 × weight of fatty acid × 100 [1.2]

Weight of alkyd – Water evolved

The 1.04 factor in the equation [1.2] converts the weight of fatty acids to the

corresponding weight of triglyceride oil.

In theory, alkyd resins are usually classified into different oil lengths (OL) or

phthalic content of the product. Percentage oil length refers to the oil portion of an

alkyd. It is equals to the weight of fatty acid in the alkyd and also weight of the polyols

needed to esterifies the fatty acid completely. The weight taken is a minus weight of

water evolved of esterification (Patton, 1962). Table 1.8 shows an approximate

classification of the alkyd resins while, Figure 1.4 shows the properties of alkyd of

different oil lengths and iodine numbers. Besides that, alkyds can be classified into

oxidizing and non-oxidizing types. An Oxidizing refers to alkyd containing drying or

semidrying oil or fatty acid. Normally, it would have 45% oil length in excess. In

contrary, non oxidizing types refer to nondrying oil or fatty acids. Hence, this type of

alkyd is used as polymeric plasticizer. Another classification is by distinguishing

between modified and unmodified alkyds. Generally, modified one contains other

monomer as an addition to polyhydric alcohol, polybasic and fatty acids as well

(Ikhuoria, Maliki et al. 2007).

43

Table 1.7: Classification of alkyd resins

Alkyd Resins Fatty Acid or Oil (%) Phthalic anhydride (%)

Short Oil Length 30-40 40-50

Medium Oil Length 40-50 30-40

Long Oil Length Over 50 20-30

Figure 1.4: Properties of alkyds of different oil lengths and iodine numbers.

44

1.6.3 Alkyd preparation process

Alkyds that are formulated from an identical chemical composition will exhibit

different properties and performance depending on the preparation process. There are

various procedures in preparing alkyds resins. In general, alkyds can be prepared from

two processes, i.e. alcoholysis (or monoglyceride) and fatty acid (Akintayo and

Adebowale, 2004).

1.6.3.1 Alkyd synthesis from alcoholysis

The alcoholysis or known as monoglyceride process is typically used when

glycerol is the choice of polyols. Oils are natural triglycerides of fatty acids. This

process involved two stage procedures. The first stage, transesterification process of oils

and glycerol must be carried out first in the presence of catalyst at temperatures 225-

250oC, before the PA is added. This process is carried as a distinct and separate step to

overcome incompatibility. In this part, polyol and glyceride oil will convert into single

homogeneous phase. At the second stage, this monoglyceride in turn provides a solvent

for polyacid which is added next to react under esterification to complete the alkyd

reaction at 180-250 o

C. This alcoholysis process is usually applied when PA is used as

polybasic acid. This is due to the fact that PA is soluble in glycerol but is not in oil.

Hence, glyceryl phthalate gel particles would be created at the early stage of the process.

Transesterification reaction resulted in a mixture of unconverted drying oil,

monoglyceride, unreacted glycerol and diglycerides. Generally, the composition

depends on the ratio of glycerol to oil, amount of catalyst and temperature. Also, the

viscosity and properties of alkyd can be affected due to extent of reaction before PA

addition.

45

1.6.3.2 Alkyd synthesis from fatty acid

Fatty acid process is chosen in this study as the preparation process of oleic acid

alkyd resin. This process involves a step-by-step addition of material to complete the

esterification at temperature 180-250°C and the method can be simplified into three

steps. Firstly, the polyhydric alcohol (glycerol) and fatty acid (oleic acid) are reacted

together simultaneously at 180 - 200°C until soft clear yellowish color resin is obtained.

In this fatty acid preparation method, there is a free-for-all competition among the -

COOH groups (all added at the beginning). However, the fatty acid -COOH groups lag

behind in joining primary -OH groups and hence must settle for connections with

secondary -OH groups. This gives rise to an alkyd with more branching being

synthesized. During the reaction, foaming occurs due to water evolved. Thereafter, the

reaction is allowed to cool at 100 °C.

In the second step, (PA) is added and heated with the resin at a moderate

temperature (120-140 °C) before increasing to higher temperature in the range 180-220

°C. Precaution should be taken to minimize the sublimation effect of PA material to the

wall of reactor, which might affect the alkyd properties. At this stage, the PA tend to

deliberately react with another primary –OH. This is significantly due to the fact that

primary hydroxyl is more reactive than a secondary hydroxyl. Anhydrides are preferred

here as a source of carboxylic functionality because the ring opening could occur at a

lower temperature. The anhydride ring opening proceeded without evolving any water

and thereafter the following esterification reaction involved releasing of water

molecules (Aydin, et al. 2004). In fact, the esterification reaction is accelerated by

efficiently removing the water evolved. Since, the esterification reaction is reversible;

the water evolved must be removed once it is formed to prevent the occurrence of

hydrolysis reaction. The third process is carried out with fumaric acid (FA). The acid is

forced to esterify with the leftover –OH group.

46

Hence, the alkyd backbone is predominantly more branched. This type of acid is

very reactive and needed to avoid heat induced cross-linking or gelation during the

alkyd synthesis. The chosen heating rate must be sufficiently high to permit the reaction

to be carried out within a reasonable time period. This is important to avoid

decomposition, discoloration, gelation and also excessive loss of volatile material. The

reaction is carried out at the required temperature until the required degree of

polymerization is reached (Velayutham et al. 2009).

The advantage of this process is the ease of operation which is much simpler

particularly when employing reactive oil and situation where oil is needed in a small

amount. In fact, the fatty acids from natural whole oils have been separated and refined.

As such, the undesired acid can be eliminated. Besides that, lower viscosity of alkyd

would be produced by this method. However, the disadvantage of this process is the cost

of fatty acids which is more expensive. The alkyd formed is more sensitive to

discolouration whereby it easily darkens under storage life. Furthermore, the fatty acids

also easily form bodying and gelation at lower acid value. There are four methods being

used to produce alkyds in which each type has some advantages over the other. The

characteristics between each process are tabulated as in Table 1.9 (Manczyk, 2002).

47

Table 1.8: Comparison of various preparation processes of alkyd

Preparation

Process

Characteristics

Acidolysis 1) Involves high temperature heating process.

2) Catalyst is used to accelerate reaction.

3) Limited to polybasic acid (e.g terepthalic/ isopthalic acid).

4) Insoluble in monoglyceride until considerable esterification.

5) Do not sublime.

Alcoholysis 1) Insoluble polyols and glyceride are converted into single

monoglycerides phase.

2) Later, reaction products act as solvent for PA which is added

to complete esterification of alkyd at T=180 - 250oC.

3) Rate of esterification is slow as high acid value is achieved.

4) Alkyd rends to be tackier and softer.

5) Alkyd tolerates more aliphatic hydrocarbon.

6) Bodying and gelation occur at high acid value.

Fatty acid/oil 1) Reaction between fatty acid and vegetable oil with polyol

and mixture dibasic acid.

2) Result a homogeneous reaction mixture due to ratio of fatty

acid to oil.

3) Overall fatty acid content in mixture is above 60-65%.

4) Alkyd produced has higher viscosity.

Fatty acid 1) Process involved a step-by-step addition of fatty acid (40-

90%) to complete esterification (T= 180 -250 oC).

2) Bodying and gelation tend to occur at lower acid value.

3) Alkyd tolerates less aliphatic hydrocarbon thinner.

4) Low acid value is more rapidly reached.

48

1.6.3.3 Process variation

Esterification process is reversible reaction; therefore, the rate of removal of

water from the reactor is an important factor affecting the rate of esterification. Alkyds

can be formed by esterification, where the reaction occurs between the –COOH and the

-OH groups of either from the same molecule or from two different compounds

according to the reaction as belows:

R-COOH + R,-OH RCOOR

, + H2O

Generally, esterification reaction involves two distinct steps as shown in Figure

1.5(i). In the first step, esterification proceeds rapidly by opening the anhydride ring in

the initial stage, which happens at lower temperature and results in the formation of

half-esters. There is no water of reaction evolved at this step. The second step shown in

Figure 1.5(ii) involving further heating by increment of the temperature produces long

chain molecules. In this step, water of reaction is evolved at a slower rate.

(i) First step

(ii) Second step

Figure 1.5: Synthesis of alkyd by the esterification process (i) first step, where

esterification proceeds rapidly by opening the anhydride ring and (ii) the second step,

where long chain molecules are produced

49

Esterification process can be conducted through two methods either in the

absence of solvent (fusion cook) or with presence of solvent (solvent cook). In the

solvent cook, the presence of solvent facilitate the removal of water of reaction,

allowing a shorter processing time being achieved without heating to excessively high

temperature. The solvent vapour serves as inert atmosphere, which prevent the ingress

of air, thus enabling light color products to be polymerized with minimum usage of inert

gas. Besides that, it dissolves the PA which can sublime on the cool portions of the

reactor, mainly in the reflux condenser and lead to its loss from the system.

The amount of solvent used in the solvent cook must necessarily be held to a

relatively small percentage of the total charge by volume (5 to 10 per cent). Also, it

should preferably be selected to have a boiling point in a range of 75 to 100 F more than

the temperature at which the alkyd is to be refluxed. Use of higher percentages of

solvent prevents the attainment of an esterification temperature. Moreover, owing to its

slow evaporation rate (only a high boiling solvent is applicable to a solvent cook), a

high percentage of the solvent in the final alkyd extends the alkyd dry time to an

intolerable degree. Most suitable solvent for this process is xylene as it has suitable

boiling point and low water solubility.

In contrary, the fusion cook technique has a flow of inert gas into reactor to

facilitate the removal of the water of reaction and avoid air ingress. During synthesis,

there is a considerable loss of volatile reactants and PA sublimation (in the reflux

condenser) during a fusion cook, particularly at higher temperature. Thus, this technique

normally had been applied in the formulation of long oil length alkyds in order to reduce

the effect on unanticipated losses on the predicted functionality (Ezeh et al. 2012).

50

1.6.3.4 Progress of reaction

The progress of reaction is monitored by periodically checking of acid number

and viscosity. Both are affected by the reaction temperature and reaction time. Figure

1.6 and 1.7 shows the relationship of the reaction temperature, time, viscosity and acid

number during the preparation of a typical medium oil linseed alkyd. Hence, the choice

of a reaction temperature is important in an alkyd synthesis. The temperature must allow

the reaction to be carried out within reasonable time period. This is so important to

reduce the operation cost, to avoid destructive decomposition, discolorations and an

excessive loss of volatile material during the reaction (Ataei et al. 2011).

Other variable that might affect acid number and viscosity of alkyds is the ratio

of reactants. The greater the ratio of hydroxyl group to carboxylic groups, the faster the

acid is reduced to low level acid number. In fact, the disappearance of carboxylic acid is

followed by titration and increase in molecular weight is followed by viscosity.

However it is critical to decide when the reaction is completed, and determination of

acid number and viscosity takes some time in sampling as the reaction is progressing in

the reactor. While the time limit is required for viscosity determination in which it

depends strongly on solution concentration and temperature, thus acid number titration

is always chosen as more appropriate method to monitor the progress of reaction.

51

Figure 1.6: Effect of esterification temperature and reaction time on viscosity

of a typical medium oil linseed alkyd

(Adapted from the Chemistry and Processing of Alkyd Resins)

52

Figure 1.7: Effect of esterification temperature and reaction time on acid value of a

typical medium oil linseed alkyd

(Adapted from the Chemistry and Processing of Alkyd Resins)

1.7 Palm oil- based polyester resin

Palm oil originates from the West Africa and was first introduced in Malaysia in

the early 20th

century. In 1917, it was commercially produced in the market. Now,

Malaysia is one of the world‘s largest producers and exporter of palm oil. Its also one

of the major oils traded in the world market. Palm oil is very special and has very wide

range of applications such in cosmetic, food manufacturing, chemical and

pharmaceutical industries as well. Its special features such as easily digestible and non-

cholesterol quality make it applicable as energy source.

53

In fact, in the non - edible products, it is also favorable to be used as base material

due to its economic superiority characteristics (Lam and Lee, 2011). In addition, the

most important non-food uses for palm products are oleochemicals. The basic

oleochemicals produced from palm oil are fatty acids, ester, alcohols, glycerin, and

sodium soap. In Malaysia, research study by Palm Oil Research Institute Malaysia

(PORIM) has led to the greatest achievement with the production of the world‘s first

palm oil-based products for the applications in industries such as automotive, furniture,

packaging and construction. The production of palm oil based polyester has led to many

advantages such as lower capital on raw material and also processing costs (Henderson

and Osborne, 2000). Table 1.9 list some fatty acids in natural oils.

The Malaysia Palm Oil Board (MPOB) was the first to produce polyols from

epoxided palm oil. Besides that, Malaysia, has successfully patented a process to

produce polyols from palm oil which was commissioned by Malaysia Palm Oil Board

(MPOB) through pilot plant. The palm-based polyols were used to produce many

applications such as insulator in the refrigerator, roof insulator, wall panels, ceiling

panels, cornices and others. Furthermore, PORIM is also looking for other applications

of palm oil based foams either rigid or flexible foams.

Continuous research, innovation and development have strategically been

conducted in order to enhance the production and create a variety of applications of

palm oil commodity (Sumathi et al. 2008).

54

Table 1.9: Some fatty acids in natural oils

Name

Formula Structure

Myristic acid

C14H28O2

CH3(CH2)12COOH

Palmitic acid

C16H32O2 CH3(CH2)14COOH

Palmitoleic acid

C16H30O2 CH3(CH2)5 CH=CH(CH2)7COOH

Stearic acid

C18H36O2 CH3(CH2)16COOH

Oleic acid

C18H34O2 CH3(CH2)7 CH=CH(CH2)7COOH

Linoleic acid

C18H32O2 CH3(CH2)4 CH=CH-CH2-CH =CH(CH2)7COOH

Linolenic acid

C18H30O2 CH3-CH2-CH=CH-CH2-CH=CH-CH2-CH =CH(CH2)7COOH

α-Eleostearic

acid

C18H30O2 CH3-(CH2)3CH=CH-CH=CH-CH=CH(CH2)7COOH

Ricinoleic acid

C18H33O3 CH3-(CH2)4CH-CH(OH)-CH2-CH=CH(CH2)7COOH

Vernolic acid

C18H32O3 CH3-(CH2)4CH-CH-CH2-CH=CH(CH2)7COOH

O

Licanic acid

C18H28O3 CH3-(CH2)3CH=CH-CH=CH-CH=CH(CH2)4(CO)-(CH2)2COOH

55

1.7.1 Alkyd resin as tackifier

Traditional tackifiers were based on naturally occurring resins such as pine tar.

There are about hundreds of tackifier grades commercially available in the market.

However, in order to select the appropriate one, an understanding of the key

characteristics of tackifiers is required. Among these, the foremost importance is their

compatibility to a specific polymer phase. Tackifiers used in modern adhesive

formulations include aliphatic and aromatic hydrocarbons, terpenes, and rosin esters.

Tackifiers are useful in PSAs or adhesives which require aggressive tack or

‗‗green strength‘‘ to assist in assembly of the product. In addition to increasing tack,

increased peel strength and decreased shear strength also result from the addition of

tackifiers in the adhesive formulation. Tackifying resins enhance the adhesion of

nonpolar elastomer by improving wettability, increasing polarity and altering the

viscoelastic properties. Dahlquist (1966) established the first evidence of modification

of the viscoelastic properties of an elastomer by adding resins and demonstrated that the

performance of PSA was related to creep compliance.

Later, Aubrey and Sheriff (1978) demonstrated that a relationship between peel

strength and viscoelasticity in natural rubber –low molecular weight resin blend existed.

In this research, palm oil based polyester resin (alkyds) were made to act as the

tackifying resin in the formulation of water based PSAs. This is because NR alone has a

very low level of attachment and adhesion to many surface types and thus requires

adding the alkyd resin. In fact, the addition would lead or encourage the required

balance of peel adhesion properties in the PSAs application. Besides that, alkyd is most

widely used as synthetic resin in coating, due to its properties such as fast dryness, good

corrosion protection, ease of application and high gloss (Leong et al. 2003).

56

Moreover, it has special characteristic of the chemical structures of ester groups

which is formed through esterification between fatty acid with polyols. Therefore, it

would allow alkyd to have versatility and diversity (Singh et al. 2012).

1.8 Environmental friendly formulation of PSAs

In general, reduction or elimination of solvents from a technology constitutes the

main possibilities for environmentally friendly products. Replacement of toxic solvents,

reduction of the solvent content, and the recycling of the solvents (and other product

components) offer other possibilities. Such changes in the manufacturing technology of

pressure sensitive adhesives (PSAs) and pressure sensitive products (PSPs) have been

developed. The development of water-based PSAs is major contribution to

environmentally friendly products and product technology (Benedek, 2009). The use of

solvent based acrylic PSAs has declined steadily for the past 10 years due to volatile

organic compound (VOC) emission control regulation, such as the Environmental

Protection Agency Clean Air Act, Title 5.

For instance, stemming from environmental concerns, developers and users of

silicone PSAs have been seeking delivery systems that eliminate or minimize VOC

content. Delivery system that is under consideration generally falls into one of the four

categories; (i) liquid solvent-less and (ii) high-solids PSAs applicable by traditional

coating techniques, (iii) aqueous PSA emulsion, (iv) hot melt PSA compositions. Each

of these delivery system choices has range of composition materials that can be used,

the final adhesive property profile obtained, and the processing equipment required

(Elliott and Glass, 2000). However, the benefits potentially reaped by the new delivery

system can be greater than solvent emission control alone. It is to be noted that due to

the composite structure of water based formulation, the narrow range of thermoplastics,

and their limited aging resistance.

57

This technological transfer leads to products with a lower environmental

resistance. Differences in recyclability also arise through formulation of water-based

dispersions. Formulation for recycling includes the use of recyclable raw material and

their recycling technology. Recyclable raw materials comprise common materials and

biodegradable products (Czech et al. 2013). Recycling of pressure sensitive products

depends on the recyclability of product components of PSA and solid state components

(carrier and release liner) of pressure sensitive product. In this study, environmental

friendly formulation was designed for making PSAs in such a way that is solvent free by

using water-based formulation. This green environmental approach is very important in

the industry for reasons of air quality, safety, and economics concern. After considering

the above factors, the idea of combining the two major primary commodities of our

country, NR and palm oil in the application of adhesive was carried on.

As we know, based on economic aspect, Malaysia was ranked as one of the

largest producer of natural rubber, largest consumer of NR and among the world‗s

largest exporters of NR products (Vaysse et al. 2012). Meanwhile, from scientific

aspect, the NR latex derived from Hevea Brasiliensis rubber tree is free of known or

suspected carcinogens, non-toxic substance and no emissions of VOCs. Besides that, it

is biodegradable, natural and renewable resources (Martín-Martínez 2002). Compared to

NR, the palm oil industry of Malaysia is also expanding significantly throughout the

past few years. As one of the major primary commodities to our country, palm oil and

natural rubber could be easily obtained and these resources are abundance in Malaysia.

58

1.9 Aims and objectives of study

The aim of this project is to develop environmental friendly pressure sensitive

adhesives (PSAs) tapes from natural rubber latex and palm oil-based alkyds. This aim

will be achieved through the following objectives.

(I) To synthesize alkyds resin derived from palm oil based polyester at three different oil

lengths by using fatty acid process and employing fusion cook technique.

(II) To characterize the properties of alkyds resin obtained by FTIR, NMR, DSC, TGA

and GPC. Other physical tests such as acid number, hydroxyl number, and moisture

content were also carried out.

(III) To prepare alkyd emulsion by employing emulsion inversion point (EIP) method

and rheological experiments were carried out to study the stability of the emulsions

obtained.

(IV) To produce a blend of stable alkyd emulsions with NR latex at a different ratios.

(V) To prepare the PSA plastic tapes by coating the blends onto corona treated

polypropylene films.

(VI) To evaluate the adhesive properties of the tapes such as peel and shear strengths by

using ASTM testing methods.

This thesis layout consists of four chapters. Chapter 1 presents an introduction

and literature review of adhesives, PSAs, alkyds resins and natural rubber. The aim and

objectives of the study were outlined. Chapter 2 provides a comprehensive description

of the experimental techniques and raw materials used throughout the study. The

procedures used to synthesize alkyd resins are described together with methods used to

characterize the properties of alkyd resin obtained. Besides that, it also describes the

procedure to prepare alkyds emulsion until the preparation of PSAs tape steps, including

ASTM method to evaluate the PSAs tape properties.

59

Chapter 3 reports the results and discussion on the results in more details such as

the influence of oil length and blending ratio towards the properties of PSAs tape

produced. Finally, Chapter 4 presents a summary and conclusions of the studies together

with some suggestions for future study.